Where do I get a Great Ball?

You need to have more than 2500 to buy a great ball. Once you get more than the amount, and you fail to catch the Pokemon using PokeBall, the game will prompt you if you would like to buy a Great Ball.

They are given as a gold purchase when you fail to capture a pokemon

To use a great ball you need 2500 coins. It appears automatically after you try with the pokeball. If you fail with the pokeball, the games offers you to use a great ball for 2500 coins and it will double the catch rate.

You don't actually 'buy' them. If the Pokemon breaks out of your pokeball AND you have at least 2500 coins, it gives you the option to use a great ball. Complete other stages or exchange jewels for more coins. You can also use the 'Check In' button once a day. I believe this gives you coins as well as updates on the game, like the Mew and Rotom special events.

After you battle Mew and if you can't catch it with the pokeball it, the greatball will appear for 2500 coins if you want it.

If you fail to catch a Pokemon with the regular pokeball it will automatically give you the option to purchase a great ball. Just make sure that you have 2,500 coins to purchase it.

You can't bye any if you miss your catch with a normal ball they will ask you if you want to use a great ball for 2500 coins plus I got Mew with 23% no great ball

The great ball will show itself when you fail to catch it with a normal pokeball, asking if you want to spent 2500 coins, to double de % catchability. So you only need the coins.

Just fight the same psychic Pokemon a few times so you train the Pokemon strong against Mew, I think thats the best way

According to the data I've received from the game if you get a certain amount of coins you can buy a great ball when you are in the middle of trying to catch a Pokemon.I'm pretty sure that's how I may be wrong though.but I recommend trying just in case I'm correct.

I believe you can only buy one for $2500 in game money if you fail at catching a Pokemon.